1. Field of the Invention
The invention relates to integrated optoelectronic modules. In particular, the invention relates to integrated optoelectronic modules forming optical systems with transmitters and receivers.
2. Description of the Related Art
An optical communication system typically includes an optical transmitter and an optical receiver. The optical transmitter includes a light source to generate light and an encoder structure to encode electrical signals into optical signals by modulating the light. The optical receiver includes a photodetector (PD) for decoding optical signals into electrical signals. Usually, the optical transmitter is more challenging to build than the optical receiver due to its packaging complexity. In most cases, the technology applied to optical transmitter modules can be easily employed in optical receiver modules. A commonly employed optical transmitter module includes a light source, a monitor PD, optical fibers, optical elements and micro-assembly structures to align the optical fibers and couple light from the light source to the optical fibers. The large dimensional difference between the emitting aperture of the light source (in the micron or sub-micron range) and the cross-section of the optical fibers (tens of microns) can result in a large mode-mismatch loss. The mode-mismatch loss is caused by two mechanisms: the mode-size mismatch and the mode-center misalignment. In addition, optical fibers usually have a small numerical aperture (NA). The NA is a parameter characterizing the range of angles over which an optical system can accept or emit light. Due to its small NA, the optical fiber only receives light incident within a small angular range.
Existing practices to resolve the optical fiber alignment problem include using micro-assembled microlens systems and self-aligned on-chip structures. The micro-assembled microlens systems can reduce the mode-size mismatch by focusing divergent laser beam into collimated light that can be coupled into optical fibers with a small NA. A drawback is that the micro-assembly packaging process requires high precision active alignment and complicated optical elements. It increases the complicity of the packaging process and cost. Self-aligned on-chip structures are usually employed to relax the active alignment requirement and packaging cost. Commonly employed solutions are focused on the self-aligned on-chip V-groove structure. This approach includes forming a reflecting mirror and V-groove alignment structure on a substrate using wet chemicals (such as KOH or TMAH) in one or multiple consecutive steps. The alignment precision is fully determined by the lithographic window opening and the solution etching rates between competing crystalline planes. Usually a few micrometers alignment precision can be achieved. Further improvement of the alignment accuracy requires precisely controlled etching time and rates between different crystalline orientations.
High density optical communication systems require high precision passive multichannel fiber alignment. Such requirement poses a challenge to currently employed approaches, especially to the final cost of the optical systems. There is a need to develop a low cost solution that requires less packaging complicity while maintain high optical performance.